Most nitrile oxides (RCNO) are little known, short-lived reactive species that are structurally isomeric with isocyanates (RNCO) and cyanates (ROCN). Being reactive, they readily undergo 1,3-dipolar additions with a great variety of multiple bond functional groups. The relative decreasing reactivity of nitrile oxides with these multiple bonds is roughly: C.dbd.S, N.dbd.N, P(V).dbd.C&gt;C.dbd.P(III), C.dbd.As, C.dbd.C, C.dbd.N, C.dbd.Se, B.dbd.N&gt;C.dbd.P, C.dbd.C&gt;P(V).dbd.N, C.dbd.N&gt;C.dbd.O. In the absence of these functional groups, most nitrile oxides will readily dimerize to furoxans (1,2,5-oxadiazole-2-oxides).
Nitrile oxides are mild oxidants and will liberate iodine from solutions of potassium iodide. The nitrile oxide function imposes the same type of solubility characteristics on a molecule that a cyano group does. Aromatic nitrile oxides, especially those where the nitrile oxide is flanked by at least one and preferably two ortho groups have been found to be stable compounds. For example, 2,4,6-Trimethylbenzonitrile oxide is a stable crystalline solid (m.p. 105.degree. C.-108.degree. C.) with two characteristic strong IR absorptions at 2290 cm.sup.-1 (--C.dbd.N) and 1334 cm.sup.-1 (--N.ident.O). In the .sup.13 C NMR, the carbon in the --CNO group is found at 35.7 ppm compared with 117.5 ppm for the carbon in the corresponding nitrile compound. This implies that the carbon atom carries considerable negative charge whereas the oxygen bears a positive charge. This highly dipolar character explains their high reactivity toward multiple bonds, see K B G Torssell, "Nitrile Oxides, Nitrones and Nitronates in Organic Synthesis," New York: VCH Publ, 1988.
The history of using polyfunctional nitrile oxides or their precursors as crosslinking agents for unsaturated polymers appears to go back to at least 1968 when chemists from Hercules were granted U.S. Pat. No. 3,390,204 for this application. In most cases, the polyfunctional nitrile oxide itself was too unstable to use directly. In these cases, the polyfunctional hydroximoyl halides were used as stable precursors. When mixed with unsaturated elastomers and then exposed to a base such as triethylamine, the hydroximoyl halide function immediately dehydrohalogenates to produce the nitrile oxide group which then rapidly crosslinks the elastomer. The resulting unfilled crosslinked elastomers from styrene-butadiene rubber (SBR), polybutadiene rubber (PBd) and natural rubber (NR) were described as hard, tough and substantially insoluble in chloroform. The crosslinking system was also noted not to be affected by air or moisture. Oddly, this patent makes no mention of any actual cured physical properties or potential applications.
Because no practical system evolved from this work, we can assume it had serious drawbacks either in process control, cost, toxicity and/or ultimate physical properties. Obviously, a two-component system requiring both a hydroximoyl halide and an organic base is not desirable, whereas the use of polynitrile oxide compounds alone is limited only to those nitrile oxides whose stability is sufficient to allow their dispersion and reaction with rubber in preference to self-dimerization. As previously mentioned, very few stable nitrile oxides are known even today. What is known about this class of chemicals is that aromatic nitrile oxides with one or two ortho substituents have enhanced chemical stability with regard to dimerization resistance. Unfortunately, the prime precursors for such nitrile oxides are sterically hindered aromatic dialdehydes. Dialdehydes of this type are relatively difficult to prepare in good yields and purity and this lack of a good synthetic method probably accounts for the field's lack of development.
In 1989, the situation began to change when Russian chemists developed a novel approach to the synthesis of sterically hindered aromatic dialdehydes (see Leonid I Belen'kii et al, Tetrahedron, Vol. 49, No 16, pages 3397-3404, 1993, and Russian Patent SU 4,750,502). With this new technique, aromatic hydrocarbons such as mesitylene or durene could be converted (in several steps) into dialdehydes in yields between 58 to 71 percent (based on hydrocarbon). This was a substantial improvement over the known technique of oxidizing bis-(hydroxymethyl) mesitylene to the dialdehyde with lead tetraacetate described by Christoph Grundmann and Reinhard Richter, "Preparation of the Dinitrile Oxide of Mesitylene," The Journal of Organic Chemistry, 33, 476 (1968).
Within a few years, another Russian group revisited the topic of rubber curing with dinitrile oxides. Only this time, they now had the dinitrile oxide of mesitylene (MDNO) readily available because of the new dialdehyde synthesis (see V V Boiko, N D Malaya and L M Klimenko, "Rheological properties of solutions of diene elastomers with the mesitylene dinitrile oxide," International Polymer Science and Technology, Vol. 20, No 10, T51, 1993). In their initial studies, they investigated the change in viscosity of various elastomer solutions as a function of time, temperature and MDNO concentration. Polymer solutions evaluated were cis-polyisoprene, polybutadiene, SBR and NBR. The relative rate of gelation was PBd&gt;SBR.apprxeq.NBR &gt;IR. With PBd and two parts of MDNO, solution gelation took place in 1.5 hours at 25.degree. C.; 0.5 hours at 40.degree. C. From this work, they concluded that MDNO could be used as an efficient low temperature vulcanizing agent for a wide variety of diene elastomers. More recent work by the same Russian group using MDNO and various diene elastomers showed the same order of reactivity as measured by Mooney viscosity increases during mixing (see V V Boiko and I V Grinev, "Influence of MDNO/Processing Elastomers," International Polymer Science and Technology, Vol. 22, No 7, T/21, 1995).
The first practical application of this technology appears to be described in Russian Patent SU 1,825,829-Al to the Ukrainian Textile Industry Research Institute. In this patent, polymethyl-vinyl-siloxane rubber having an M.W. of 500,000 and a molar olefin content of 0.45-0.55 percent was crosslinked with MDNO in ethyl acetate on a fabric.
The crosslinked fabric treatment produced a durable water and dirt repellent finish. Russian Patent SU 1,824,389-Al has also been issued to the Ukrainian Textile Industry Research Institute for the synthesis of 2,4,5-trimethylbenzene-1,3-dialdehyde as an intermediate to the corresponding dinitrile oxide as a low temperature hardener. Russian Patent 2,042,664-C1 describes a new synthesis for dialkylbenzene dinitrile oxides and demonstrated their utility for curing rubbers with low levels of unsaturation at low temperature. Whereas, MDNO has its nitrile oxide groups each flanked by two methyl groups, the new dinitrile oxide compounds have each --CNO group flanked by only one alkyl group. It is not clear, however, whether or not this structural difference would result in different rates of cure since a comparison control with MDNO was not included in the cured rubber physical property data. Nevertheless, a comparison of the room temperature cured properties of several low unsaturation polymers with the same polymers cured with conventional high temperature sulfur or peroxide systems, showed remarkable similarity.
The starting materials for this new synthesis are meta or para-dimethyl- or meta or para-diethylbenzene. The hydrocarbons are treated with specific molar ratios of aqueous formaldehyde solution, hydrochloric acid, sulfuric acid and acetic acid at 70.degree. C.-85.degree. C. to prepare bis-(chloromethyl)-dialkylbenzenes (see Milton J Rhoad and Paul J Flory, "Preparation of Bischloromethylmesitylene," Journal of the American Chemical Society, 72, 2216 (1950)). After cooling to 15.degree.C.-25.degree. C., crystals of the bis-chloromethyl compounds are then filtered off, washed with water and dried. In the next step, the bis-chloromethyl compounds are treated with an aqueous solution of hexamethylenetetramine in acetic acid for 4 hours at 95.degree. C.-100.degree. C. The organic products (dialdehydes) were then extracted from the reaction mixture with carbon tetrachloride. After water-washing the carbon tetrachloride solution, an aqueous solution of hydroxylamine hydrochloride was added followed by the addition of an aqueous NaOH solution to generate free hydroxylamine.
The organic layer was then separated after about 1 hour at 25.degree. C.-35.degree. C. The aqueous layer was cooled to 20.degree. C. and neutralized to pH 7 with HCl to precipitate the dioxime. The dioxime is filtered off, washed with water and dried.
Treatment of the dioxime with acidified sodium hypochlorite solution at -10.degree. C. to 0.degree. C. gave the crystalline dinitrile oxide compound in very good yields.
Although this synthesis is a great improvement over previous methods, it does have several drawbacks such as the use of a chlorinated solvent and multiple extractions. Additionally, the hexamethylenetetramine/acetic acid method for conversion of the chloromethyl group into an aldehyde is not synthetically useful for the conversion of more highly hindered halomethyl groups such as those found in halomethylated mesitylene.
A W van der Made and R H van der Made, "Preparation of Bromomethylaromatic Compounds," The Journal of Organic Chemistry, 58, 1262 (1993) reports that mesitylene and other structurally similar aromatic compounds can be efficiently bis-bromomethylated in high yields using readily available reagents. This technique allows more efficient access to bis-bromomethyl compounds which can subsequently be converted into dinitrile oxides by employing prior art techniques.